1. Field of the Invention
The invention relates to magnetic bearings, and more particularly, to magnetic bearings for the triaxial position stabilization of bodies.
2. Description of the Prior Art
Magnetic bearings on opposite ends of a movable bearing part, typically have fixed bearing parts. Between the fixed bearing parts, a magnetic flux is maintained penetrating the movable bearing part in one direction. For the production of retaining forces parallel to the magnetic flux direction, electric coils are installed on the fixed bearing parts, which are controlled by a sensor system which measures the position of the movable bearing part in a contact-less manner to generate an error signal which then is fed to a servomechanism feedback circuit which adjusts the position of the movable bearing part.
Magnetic bearings of this type are known, for example U.S. Pat. No. 3,860,300 and German Patent No. DT-PS 24 44 099. Such bearings are used particularly for the axial stabilization of rotors in magnetic bearings. Refer, for example. to Voss-Cohen, "UHV compatible chopper system" in J. Vac. Sci. Technol., 1980, Vol. 17, No. 1, page 303 ff., and Fremerey/Boden "Active permanent magnet suspensions for scientific instruments" in J. Phys. E.: Sci. Instrum., 1978, Vol. 11, page 106 ff. The advantage of these known permanent magnetic rotor bearings resides in the fact that, for purposes of operating a contact-less bearing manner on all sides of the rotor, they required only a stabilization in the direction of the rotor axis. This advantage, however, is attained only at the expense of the disadvantage, also known, that such bearings exhibit practically no damping in the radial directions. The problems which result when critical rotor speeds are passed through can be countered, to a limited extent, with an increased expenditure, by careful balancing of the rotor system, as described by Voss/Cohen in their above-referenced publication. It is also known that additional electronic or mechanical damping devices can be used, to reduce the disruptive effect of vibrations on the rotor bearing. See Fremerey "Spinning rotor vacuum gauges" in Vacuum, 1982, Vol. 32, No. 10/11, page 685 ff. All the above-cited patents and publications are incorporated herein by reference.
For the stabilization of magnetic bearings, eddy-current damping devices are also used. Thus, in U.S. Pat. No. 3,929,390, the attachment of fixed copper discs to the end surfaces of permanent magnets fastened to rotating parts therein is proposed, to stabilize a bearing system. Such a damping apparatus has a low degree of efficiency in relation to the amount of permanent magnetic material used, because at the free ends of the permanent magnets, the magnetic field produced by the permanent magnets diverges strongly, and thus the magnetic field components, required for the desired radial eddy-current damping, have only a small penetration into the copper discs in the axial direction.
Significantly higher efficiencies are achieved by the installation of fixed copper discs in the field between two permanent magnets connected in series behind one another (See Report ESA-CR (P)-696, MU/EX No. 47.055/75, page 12, which is incorporated herein by reference. In this apparatus, the magnetic fields run inside the copper essentially in the axial direction, so that there is an optimal utilization of the field for the eddy-current damping of radial rotor movements. The effort and expense involved, however, are considerable. A total of 6 annular permanent magnets are required, 2 of which must also exhibit the radial magnetization direction, which is difficult to achieve from a manufacturing point of view. Considerably simpler, in the design configuration of its magnetic circuit regarding the efficiency achieved, is the radial eddy-current damping of a magnet system suspended on threads, described by Fremerey in "High vacuum gas friction manometer" in J. Vac. Sci. Technol., 1972, Vol. 9, No. 1, pp 108 ff which is incorporated herein by reference. Here, a fixed copper disc is penetrated by a magnetic field running axially between the end surface of a permanent magnet and a flat iron disc. On this apparatus, however, the coupling of the eddy-current damping apparatus to the body supported in a contact-less manner is very difficult and expensive. For this purpose, electronic amplifiers with multi-element sensor coils and electromagnetic deflection coils are necessary which are disposed in two directions independent of one another.
The last two devices described above, in addition to the indicated expense and complexity, have the disadvantage that they can only be used for radial damping. Further, they do not represent magnetic bearings.
In Sabnis, Dendy and Schmitt, "A Magnetically Suspended Large Momentum Wheel," J. Spacecraft, July 1975, Vol. 12, No. 7, pp. 420 ff., which is incorporated herein by reference, a three-loop magnetic bearing is shown where bias flux is provided by a stationary ring magnet. This flux is lead by the structure of the bearing across four axial gaps. Passive radial stiffness is provided through the action (minimum reluctance) of opposed concentric rings at their air gaps, the total stiffness being proportional to the number of rings. Radial damping is provided at least in part by conducting material, such as copper wire, placed in the inter-ring grooves at the air gaps. This bearing requires a complex, intricate and heavy ferromagnetic structure attached to the bearing shaft which is expense to manufacture. As the bearing gaps are formed between iron pole pieces, the bearing structure suffers a considerable unbalance stiffness along the axial direction. Further, the efficiency of the damping is rather low because of the limited amount of conducting material which can be placed in the relatively small inter-ring grooves.
The eddy-current damping apparatus described above according to U.S. patent application Ser. No. 3,929,390 uses, for damping, the permanent magnets of the radial bearing, but for the reasons mentioned above it has only a low degree of efficiency. The required magnetic bearing's axial bearing is located elsewhere.